Brightway2: Including weighting and normalization

Weighting and normalization are two optional steps in life cycle impact assessment calculations. Until now, there has not been any support for anything other simply multiplying a life cycle inventory by a vector of characterization factors in Brightway2. This wasn't a problem, because the ecoinvent centre implementation of its provided impact assessment methods already included weighting and normalization steps where appropriate. However, we would like to be able specify, and modify, these values ourselves.